1. Field of the Invention
The present invention relates generally to a heterojunction bipolar transistor having a modulation doped base. More particularly, the subject invention pertains to a heterojunction bipolar transistor wherein the base is a modulation doped heterojunction formed of a layer of p wide gap material and a layer of I narrow gap material. The collector/base is formed with n+ doped narrow gap semiconductor material adjacent to the I region of the base. The emitter is n+ doped wide gap semiconductor material adjacent to the p wide gap layer of the base. The doping concentration and width of the p wide gap region of the base is such that all holes are depleted in that region, and a p type inversion layer is formed in the I narrow gap material at its interface with the p wide gap material.
2. Discussion of the Prior Art
Two major factors adversely affect the performance of modern bipolar transistors, namely: 1) reverse injection of minority carriers at the emitter/base junction and 2) increased base resistance for narrow base widths. The reverse injection of minority carriers at the emitter/base junction can be overcome by a wide energy gap emitter. The increased base resistance for narrow base widths can be overcome by heavy doping of the base region. However, a heavily doped base suffers from severe impurity scattering which does not reduce the base resistance commensurate with the doping concentration, and the problem becomes more severe at low temperatures because of increased impurity scattering cross section.